International Journal of the Physical Sciences Vol. 6(14), pp. 3319-3329, 18 August, 2011 Available online at http://www.academicjournals.org/IJPS DOI: 10.5897/IJPS10.274 ISSN 1992 - 1950 ©2011 Academic Journals

Full Length Research Paper

Geoelectrical studies for estimating aquifer hydraulic properties in State,

Chukwudi C. EZEH

Department of Geology and Mining, University of Science and Technology, Enugu, Nigeria. E-mail: [email protected]

Accepted 26 May, 2011

Three hundred and twenty two vertical electrical soundings (VES) have been used to evaluate the hydraulic properties of aquifers in Enugu State, Southeastern Nigeria. The project domain lies within longitudes 7° 6’ E to 7° 54’E and latitudes 5° 56’N to 6 °52’N, and covers an area of about 7161 km 2 over eight main geological formations. The thickness, lateral extent and resistivity of the aquiferous layers were determined by the electrical survey. Also, zones of high yield potentials were inferred from the resistivity information. Transmissivity values were inferred using the empirical relationship between hydraulic conductivity and formation factor. Results show highly variable thickness of the main aquifer in the study area. Aggregate longitudinal conductance indicates greater depth of the substratum in the central part of the area, underlain by the Ajali and Formations. High values of transmissivity, specific yield and moderate porosity also predominate, thus suggesting thick and prolific aquiferous zone. Lower average values of these parameters were obtained in southwestern and eastern parts underlain by the Imo and Ezeaku/ AwguNdeaboh/ Nkporo Formations, with the eastern part recording the lowest values, except porosity with the highest estimated average value. Maps of various geoelectrical and hydraulic parameters have been produced and the overall results can serve as a useful guide in planning a drilling program in the study area.

Key words: Formation factor, transmissivity, porosity, specific yield.

INTRODUCTION

Knowledge of hydraulic properties of subsurface aquifers In recent years, attempts have been made by several is essential for the determination of natural flow of water authors to obtain hydraulic parameter estimates from through an aquifer and groundwater assessment. resistivity soundings (Kelly, 1979; Koinski, 1981; Mazac Hydraulic conductivity (k), transmissivity (T) and and Kelly, 1985). Transmissivities, formation factors and storativity (S) are aquifer properties that may vary specific yield have been estimated using empirical and spatially because of geologic heterogeneity. Estimation of semi-empirical relationships (Schimscal, 1981; Frohlick these properties allows quantitative prediction of the and Kelly, 1985; Huntly, 1986; Urish, 1987; Chen et al., hydraulic response of the aquifer to recharge and pump. 2001; Singh, 2005). Although previous attempts in Transmissivity, the hydraulic conductivity multiplied by relating geoelectrical and hyrogeological parameters the saturated thickness of the aquifer, represents a have been made, the empirical relations obtained were vertical average of hydraulic conductivities that may vary mostly applicable to areas with uniform geology. There is with depth. Most of the analytical techniques used to therefore the need to make these relations more general estimate the hydraulic properties of aquifer were in nature, so that they can be more widely applied in developed for porous media, such as unconsolidated areas with diverse lithological characteristics. sediments. Although these properties are mainly deduced In the present study, an attempt has been made to find from pumping test analysis, geophysical methods now a general functional relationship between surface provide an effective technique for aquifer evaluation and resistivity depth soundings and hydraulic parameters in can greatly reduce the number of necessary pumping the study area. Hydraulic conductivity obtained from well tests, which are both expensive and time consuming. tests is correlated with formation factor. Model calculations 3320 Int. J. Phys. Sci.

NIGER

BENIN

STUDY AREA

CAMEROUN ATLANTIC OCEAN

Figure 1. Map of Nigeria showing the location of the study area (World Gazette, 2011).

have been used to obtain regional patterns of Geology transmissivity, porosity and specific yield. The study area is underlain by the following geological formations (Figure 3), the Asu River group, Eze Aku Description of study area Shale group, /Ndeabor Shale group, Nkporo Shale, Mamu Formation, Ajali Formation, Nsukka Formation and Physiography Imo Formation. The Asu River group is the earliest recorded marine The study area (Figure 1) shows two major types of sediments consisting of bluish grey to brown shale and landforms which consists of a high relief central zone with sandy shale, fine-grained micaceous sandstones and undulating hills and ridges and the lowland area (Figure dense blue limestone (De-Swardt and Casey, 1963; 2). Both are related to the geology of the area. Reyment, 1965). The high relief zone is geologically associated with the The sediments of the Eze Aku formation consist of outcrops of Ajali Sandstone and Nsukka Formation, while fossiliferous limestone and shale. The thickness varies, the eastern lowland zone is associated with outcrops of but may attain 1000 m in places. The Awgu/Ndeaboh Asu River group, Eze Aku Shale group, Awgu/Ndeabor Formation is about 400 m thick and consists of bluish shale group, Asata/Nkporo Shale group and parts of grey, well-bedded shales, with subordinate calcareous Mamu Formation. The western lowland zone is sandstones and limestones (Kogbe, 1981). The Nkporo associated with outcrops of the Imo Formation. On the Formation comprises dark shales and mudstones, with scarp face, slope failures, land slides, soil and gully occasional thin beds of sandy shale and sandstone and erosion and slump features are common. In general thickness of about 150 m. The Owelli sandstone, Enugu terms, the Ajali Sandstone usually underlies areas of Shale and Asata shales are lateral equivalents of the height above 300 m while the Nsukka Formation is Nkporo Shale. The Owelli sandstone comprises medium characterized by abundant residual hills. to coarse-grained sandstones with pebble bands while Ezeh 3321

METERS 500 480 460 440 420 N 400 380 360 340 320 *IKEM 300 *NSUKKA 280 260 400 240 220 200 180 200 *UDI *AGBANI

ELEVATION 160

*ADANI 140 120 *OJI RIVER *AWGU *UMULOKPA 100

VERTICAL SCALE: 1 : 200

HORIZONTAL SCALE: 1 : 850 000

Figure 2. Surface map of the study area.

the Enugu/Asata shales consists of soft dark grey shales formation constitute outliers and its thickness averages and mudstones with occasional thick beds of white 250 m. sandstones and sandy shales. The Imo information consists dominantly of blue to dark The Mamu formation consist of fine to medium grained, grey shales, with occasional bands of clay- ironstone and white to grey sandstones, shaly sandstones, sandy subordinate thin sandstones. The formation includes thick shales, grey mudstones, shales and coals. The thickness sandstone units at several horizons and rests is about 450 m and it conformably overlies the Enugu conformably on Nsukka Formation with a thickness of shale. about 500 m. Quaternary alluvial deposits overlie the The Ajali Formation, also known as false bedded Northwestern parts of the area. These areas are within sandstone, consists of thick friable, poorly sorted the Anambra River flood plain and are generally less than sandstones, typically white in colour but sometimes iron- 100 m in altitude. stained. The thickness averages 300 m and is often overlain by considerable thickness of red, earthy sands, formed by the weathering and ferruginization of the Hydrogeology formation. The Nsukka Formation lies conformably on the Ajali The hydrologic units in the study area include confined, Sandstone. The lithology is very similar to that of Mamu semi-confined, unconfined, perched and fractured shale Formation and consists of an alternating succession of aquifers. sandstone, dark shale and sandy-shale, with thin coal Confined conditions exist over the Ajali Sandstone in seams at various horizons. Eroded remnants of this areas overlain by the Nsukka Formation and/or the Imo 3322 Int. J. Phys. Sci.

7.1

LEGEND 7 BENUE STATE KOGI STATE ALLUVIUM

6.9 +NSUKKA IMO SHALE *NSUKKA GROUP 6.8 NSUKKA *IKEM FORMATION *ADANI 6.7 *UKEHE AJALI SANDSTONE 6.6 EBONYI STATE MAMU *UMULOKPA FORMATION 6.5 ASATA/ NKPORO SHALE *ENUGU 6.4 AWGU/ NDEABOR SHALE GROUP 6.3 *UDI *AGBANI EZEAKU SHALE GROUP 6.2 *OJI RIVER ASU RIVER GROUP 6.1 *AWGU VES LOCATION 6 BOREHOLE LOCATIONS

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9

KM 0 8.5 17 25.5

Figure 3. Geologic map of the study area showing VES and borehole locations.

Formation, and in the Mamu Formation where the dug holes gave thickness values ranging from 3 to 8 m overlying Ajali Sandstone and Nsukka Formation are with an average of about 4.6 m (Uma, 2003). considerably reduced in thickness or eroded. Semi- Fractured and fissured shale aquifers exist mostly in confined situation exist in places and usually comprise the Awgu/Ndeabor and Enugu/Nkporo Shale groups interbedded thick sequence of sand (aquifer) and sandy where recurrence fractures and weathering result in clay or clayey-sand aquicludes. Various aquifers in this economic water yield in the upper unites of the shales. group occur in the upper to middle horizons of Ajali Sandstone and in the upper section of the Mamu Formation and constitute the partial recharge zones for Basis of estimating hydraulic properties from the deeper-seated confined aquifers (Egboka and resistivity Onyebueke, 1990; Akudinobi and Egboka, 1996). Unconfined aquifer units in the study area occur mostly in A clear analogy exists between mathematical the Ajali Sandstone, and represent sections of the descriptions of the process of groundwater flow and formation where the semi-permeable or impermeable cap electrical transmission. The electrical current flow (J) in a beds have either been eroded or absent. The thickness conducting medium is governed by Ohm’s law and the of these aquifer units vary from shallow to deep in places. groundwater flow in a porous medium, by Darcy’s law, Perched aquifer conditions occur mostly in the both having forms of equation: lateritic/red earth cover over the Nsukka Formation and in σdv the upper sandy units of the Nsukka Formation. The J = - (1) perched aquifer is generally thin and measurements in dr Ezeh 3323

q = - kdh (2) were carried out in one hundred and twenty six locations within the dr study area (Figure 3). The Schlumberger electrode spreading was used with maximum current electrode separation ranging from 400 m to 1.2 km. Most of the soundings were conducted nearby existing Where j, , v, r, q, k, h are respectively the current boreholes for correlation purposes. The locations of the boreholes density (amps per unit area), electrical conductivity are also shown in Figure 3. (siemens/m = reciprocal resistivity, ℓ, ohm-m or m), The initial interpretation of the VES data was accomplished using electrical potential (volts), distance (metres) specific the conventional partial curve matching technique, with two-layer discharge (discharge per unit area) hydraulic conductivity master curves in conjunction with auxiliary point diagrams (Orellana and Mooney, 1966; Koefoed, 1979; Keller and Frischknecht, 1966). (or permeability; m/s) and hydraulic head (m). The From this, estimates of layer resistivities and thicknesses were electro-hydrological analogy between these two obtained which served as starting points for computer-assisted phenomena is widely accepted (Freeze and Cherry, interpretation. The computer program OFFIX, was used to interpret 1979; Fitts, 2002; Singh, 2005). all the data sets obtained. From the interpretation of the resistivity In a homogeneous and isotropic medium, both the data, it was possible to compute, for every VES station, the electric current and groundwater flow satisfy the Laplace longitudinal conductance. equation: for electrical flow, S = hi /li (8) 2 2 d v dv = 0 (3) 2 + and the transverse resistance, dr r dr and for groundwater flow, R = hili (9) d 2 h 1 dh where hi and li are layer thickness and resistivity respectively + = 0 (4) dr 2 r dr (Maillet, 1947). Further quantification of geoelectrical depth sounding results was possible by relating layer resistivity to lithological, and pore water For a point current source, the solution of Equation (3) in properties. The starting point was the Archie’s Law (1942) which a semi-infinite, homogeneous medium for (hemispherical relates the layer resistivity derived from geoelectric field curve to earth) electrical flow can be written as: pore water resistivity and the porosity and cementation of the porous skeleton: lI 1 V = (5) la 2π r F = (10) lw and for hydraulic flow a similar equation can be written as l where a, is the resistivity of the saturated rock, Q ℓw is the water resistivity and F is the resistivity formation factor h = Inr (6) which is constant for pure sands. According to Archie (1942), the 2πT formation factor is related to porosity ( φ) by

Transmissivity of an aquifer of saturated thickness b, is F = a φ-m (11) then expressed by where a and m are constants related to rock type.

Tr = kb (7) After a soil with porosity Φ has been drained, its volumetric moisture content is equal to its specific retention. The unsaturated Such that equation 4 becomes zone is assumed to represent gravity-drained aquifer material at specific retention. If the saturated zone consists of the same h = Q Inr (8) material at 100% saturation, then specific yield is defined by: 2 π kb Sy = Φ (1 – Sr) (12)

Generally, since larger connected pores make better flow where Sr is saturation at specific retention. Achie (1942), combined characteristics for both water and electric current, it is Equations 10 and 11 to relate the bulk resistivity to porosity φ, the expected that at the very least, there should be some pore fluid resistivity ℓw and the cementation factor m, as: relationship between electrical and hydraulic parameters -m (Singh, 2005). ℓsat = ℓw φ (13)

The resistivity of the unsaturated soil is then METHODOLOGY -n ℓunsat = ℓsat Sr . (14) Data acquisition and interpretation From Equations 13 and 14, porosity and saturation can be Three hundred and twenty two vertical electrical soundings (VES) expressed in terms of resistivities to give specific yield S y as: 3324 Int. J. Phys. Sci.

30 K=1.15F-0.84 (R=0.91) 25

20

15

10

5

Hydraulic Conductivity K(m/day) Conductivity Hydraulic 0 0 5 10 15 20 25 30

Formation Factor (F)

Figure 4. Plot of hydraulic conductivity versus formation factor.

1  1  transmissivity values were compared with estimates of Egboka and  l m   l n  Uma (1985), Onuoha and Mbazi (1988), Ezeigbo and Ozoko (1989)  w  1− sat  and Akudinobi and Egboka (1996). The results were considered Sy =       (15) satisfactory thus validating the applicability of the method.  l   l  sat  unsat 

RESULTS AND DISCUSSION ℓsat and ℓunsat are obtained from geoelectrical depth sounding, while ℓw is the water resistivity. In the present study, m was assigned values between 1 and 1.2 for shale and 1.5 for sand; n is assumed Regional maps of aquifer thickness, longitudinal to be 2 (Keller and Frischknecht,1966) ℓa was obtained from VES conductance transmissivity, porosity and specific yield sounding results while ground water resistivity ℓw was determined have been constructed using the results of the resistivity from electrical conductivity measurements at wells and boreholes soundings interpretation. well distributed in the study area. The water conductivity σw, -1 Aquifer thickness is highly variable in the study area measured in µmhos cm , was converted to resistivity ( Ωm) with the relation: (Figure 5). In the central part, underlain by the Ajali and Nsukka Formations, the thickness range between 45.12 4 -1 ℓw (Ωm) = 10 /σw (µmhos cm ) (16) and 206.17 m with an average value of 105.20 m. Depths to the top of potential aquifers range from 12 to 40 m in these areas. In the southwestern part of the study area Estimating transmissivity underlain mostly by the Imo Formation, aquifer thickness range between 31.92 and 204.18 m with an average Figure 4 shows the plot of hydraulic conductivity (K) against formation factor (F) estimated from Equation 10. Through a value of 94.11 m, while the range is between 4.3 and representative average of hydraulic conductivity values obtained 71.2 m with an average of 33.78 m in the eastern part from pumping test analysis in parts of the study area, the following underlain by the Eze Aku/Awgu Ndeaboh/Nkporo empirical relation between K and F was obtained using linear Formations. Potential aquifers occur at depths ranging regression techniques with a correlation coefficient, R = 0.91. from 30 to 56 m in the southwestern part and from 2 to 24

K (m/day) = 1.15F - 0.84 (17) m in the eastern part of the study area. The distribution of the aquifer raw longitudinal The hydraulic conductivity k, obtained from Equation 17 at each conductance computed from the resistivity sounding sounding position and the aquifer thickness, b, resulting from interpretation for the entire area of study is shown in multilayer resistivity models, were used to derive the transmissivity Figure 6. Minimum longitudinal conductance values were Tr, according to Equation 7: observed in the central portion (Nsukka-Ukehe-Oji River

axis) of the study area, underlain by the Ajali and Nsukka Tr = Kb = (1.15F-0.84) b (18) Formations. Maximum values were observed in the In order to access the reliability of the method, the estimated eastern part of the study area underlain by the Eze- Aku/ Ezeh 3325

7.1

7

KOGI STATE BENUE STATE METERS 6.9 210 NSUKKA 200 6.8 190 180 IKEM 170 6.7 ADANI 160

UKEHE 150 EBONYI STATE 140 6.6 130 UMULOKPA 120 110 6.5 100 90 +ENUGU 6.4 80 70 60 6.3 ANAMBRA STATE UDI AGBANI 50 40 30 6.2 20 OJI RIVER 10 0 6.1

+AWGU ABIA STATE 6

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 KM 0 8.5 17 25.5

Figure 5. Isopach map of the aquiferous layer.

Awgu Ndeaboh/Nkporo Formations, thus indicating a area are shown in Figures 8 and 9. In the central part of highly conductive environment. the study area underlain by the Ajali and Nsukka Transmissivity values have been estimated using the Formations, porosity values range from 13.27 to 42.5% Equation 18. Zones of high transmissiviity values are with an average value of 25.12%, while values for concentrated on the central part of the study area, specific yield range between 1.49 and 25.67% with an underlain by the Ajali and Nsukka Formations, where the average value of 10.71%. In the western part underlain value ranges from 317.32 to 2449.977 m2/day, with an by the Imo Formation, porosity range between 16.56 and average value of 1163.95 m2/day while average values of 57. 66% with an average value of 29.92%, while specific 771.38 and 120.47 m 2/day were recorded in the western yield values range from 5.2 to 38.86% with an average and eastern parts of the study area underlain by the Imo value of 14%. Higher porosity and lower specific yield and Eze-Aku/Awgu Ndeaboh/Nkporo Formations values were recorded in the eastern part, underlain by respectively. Generally, transmissivity of the aquifer in the the Eze Aku/ Awgu Ndeaboh/ Nkporo formations. They study area has a wide range, from very poor to very ranged from 34.29 to 80.23% with an average of 59% prolific, improving from east and west to the central part and from 0.23 to 6.4% with an average value of 2.87% of the study area. On the regional scale also, the quality respectively. deteriorates towards the discharge area. Relating the These range of porosity and specific yield values in the transmissivity distribution (Figure 7), and the geology different geologic formations within in the study area (Figure 3) it is evident that T r values are highest in the mostly underlain by sandy and shaly units, is consistent areas underlain by Ajali/Nsukka Formations, consisting with the range of values for sand and shale given by lithologically of mainly sands. Davis and DeWiest (1966), Freeze and Cherry (1979) The variation of porosity and specific yield in the study and Pettijohn (1975). 3326 Int. J. Phys. Sci.

7.1

7

BENUE STATE SIEMENS KOGI STATE 6.9 2.8 +NSUKKA 2.6 6.8 2.4 +IKEM 2.2 6.7 +ADANI 2 +UKEHE 6.6 EBONYI STATE 1.8

+UMULOKPA 1.6 6.5 1.4 1.2 +ENUGU 6.4 1 0.8 ANAMBRA STATE +AGBANI 6.3 +UDI 0.6 0.4 6.2 0.2 +OJI RIVER 0 6.1 +AWGU ABIA STATE 6

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 KM 0 8.5 17 25.5

Figure 6. Longitudinal conductance map of the study area.

Relation between groundwater resistivity and earth This empirical relationship between apparent resistivity resistivity and water resistivity reveals that the earth resistivity is strongly affected by groundwater electrical conductivity The relationship between water quality and earth and provides a basis for applying resistivity methods in resistivity was determined by plotting the water resistivity groundwater quality evaluation especially in the coal as a function of apparent resistivity, using results of mining areas of the state. vertical electrical soundings (VES) conducted nearby productive boreholes well distributed in the area where electrical conductivity (EC) values were available. Conclusions The fitted line between water resistivity and earth resistivity (Figure 10) indicates the following empirical An attempt has been made to find a more general relationship with correlation coefficient of 0.89: functional relationship between hydraulic conductivity and resistivity in the study area. The established analytical ℓw = 0.074 ℓa + 90.93 (19) relationship between formation factor and hydraulic conductivity has been used to estimate and construct where ℓw is the water resistivity in ohm–m, and ℓa is the regional map of transmissivity. Also, geoelectrical depth apparent resistivity in ohm-m. soundings in combination with water tests have been Ezeh 3327

7.1

7

KOGI STATE BENUE STATE PERCENT 6.9 58 +NSUKKA 56 6.8 54 52 +IKEM 50 6.7 +ADANI 48 46 +UKEHE 44 42 6.6 EBONYI STATE 40 +UMULOKPA 38 36 6.5 34 32 +ENUGU 30 6.4 28 26 ANAMBRA STATE 24 +UDI +AGBANI 6.3 22 20 18 6.2 16 +OJI RIVER 14 12 6.1 +AWGU

ABIA STATE 6

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 KM 0 8.5 17 25.5

Figure 7. Transmissivity map of the study area.

7.1

7

BENUE STATE KOGI STATE M^2/DAY 6.9 2500 +NSUKKA 2400 2300 6.8 2200 2100 +IKEM 2000 6.7 +ADANI 1900 1800 +UKEHE 1700 6.6 EBONYI STATE 1600 1500 +UMULOKPA 1400 1300 6.5 1200 1100 +ENUGU 1000 6.4 900 800 700 6.3 ANAMBRA STATE +UDI +AGBANI 600 500 400 300 6.2 200 +OJI RIVER 100 0 6.1 +AWGU ABIA STATE 6

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 KM 0 8.5 17 25.5

Figure 8. Map of porosity in percent. 3328 Int. J. Phys. Sci.

7.1

7 KOGI STATE BENUE STATE PERCENT 6.9 34 +NSUKKA 32 6.8 30 +IKEM 28 +ADANI 6.7 26 +UKEHE 24 6.6 EBONYI STATE 22 20 +UMULOKPA 18 6.5 16 +ENUGU 14 6.4 12 10 6.3 ANAMBRA STATE +UDI +AGBANI 8 6 4 6.2 +OJI RIVER 2 0 6.1 +AWGU

ABIA STATE 6

6.9 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 KM 0 8.5 17 25.5

Figure 9. Map of specific yield in percent.

900 ℓw=0.074 ℓa+90.93[R=0.89] 800 700 600 500 400 300 200

Water Resistivity (Ohm-m) Resistivity Water 100 0 0 2000 4000 6000 8000 10000 Apparent Resistivity (Ohm-m)

Figure 10. Plot of water resistivity versus apparent resistivity.

utilized to construct regional maps porosity and specific electrical conductivity values can be made in areas where yield. The apparent resistivity/ water resistivity correlation wells have not been drilled. relationship approximated to a linear function with signi- The various geophysical and hydraulic parameters ficant correlation coefficient within the range of values estimated and presented here will no doubt aid in used. Using this relationship, preliminary estimates of borehole programs and provide additional database for Ezeh 3329

groundwater development and utilization in the study Kogbe CA (1981). Attempt to correlate the statigraphic sequence in the area. The results have therefore shown that it is possible middle Benue basin with those of the Anambra and upper Benue basins. Earth Evolution Sci., 1(2): 139-148. to obtain quantitative results from VES that are useful for Koinski WK, Kelly WE (1981). Geoelectric soundings for predicting the determination of hydraulic properties of aquifers. aquifer properties. Ground Water. 19 (2): 163-171. Maillet R (1947). The fundamental equations of electrical prospecting. Geophysics. 12: 529-556. Mazac O, Kelly WE (1985). A hydrogeophysical model for relations REFERENCES between electrical and hydraulic properties of aquifers. J. Hydrol., 79:

1-19. Akudinobi BEB, Egboka BCE (1996). Aspects of hydrogeological Onuoha KM, Mbazi FCC (1988). Aquifer transmissivity from electrical studies of the escarpment regions of southeastern Nigeria. Water sounding data: the case of Ajali sandstone aquifers southwest of Resour. J. NAH. 7 (1 & 2): 12 - 25. Enugu, Nigeria. In: Ofoegbu CO Ed. Groundwater and mineral Archie GE (1942). The electrical resistivity log as an aid in determining resources of Nigeria. Vieweg – Verlag. pp. 17 – 30. some reservoir characteristics. Trans. Am. Inst. Metal Petr. Eng. 146: Orellana E, Mooney HM (1966). Master tables and curves for vertical 54 – 62. electrical sounding over layered structures. Interscience Madrid. Chen J, Hubbard S, Rubin Y (2001). Estimating the hydraulic Pettijohn FI (1975). Sedimentary rocks 3rd Ed. Harper & Row Publ. conductivity at the south Oyster site from geophysical tomographic New York. data using Bayesian techniques based on the normal linear Reyment RA (1965). Aspects of the geology of Nigeria. Ibadan regression model. Water Resour. Res., 37 (6): 1603 - 1613. University Press. Davis SN, DeWiest RJM (1966). Hyrogeology. John Wiley and Sons, Schimschal U (1981). The relationship of geophysical measurements to Inc. hydraulic conductivity at the Brantley dam site, New Mexico. De-Swardt AM, Casey OP (1963). The Coal resources of Nigeria. Bull. Geoexploration. 19: 115 – 125. Geol. Survey of Nigeria, p. 28. Singh KP (2005). Nonlinear estimation of aquifer parameters from Egboka BCE, Onyebueke FO (1990). Acute hydrogeological problems surficial resistivity measurements. Hydrol. Earth Sys. Sci. Discuss., Vis-à-Vis- planning and management of a developing economy: A 2: 917 – 938. case study of Enugu area, Nigeria. Water Res. J. NAH 2(1): 43 – 55. Uma KO (2003). Hydrogeology of the perched aquifer systems in the Egboka BCE, Uma KO (1985). Acid mine drainage problems of Enugu hilly terrains of Nsukka town, Enugu State, Nigeria.Water Resour. J. Coal mines of Anambra State Nigeria. Proc. Int. Mine Water NAH. 14: 85 - 92. Congress Granada, Spain. I: 1 – 11. Urish DW (1987). Electrical resistivity-hydraulic conductivity Ezeigbo HI, Ozoko DC (1989). An evaluation of the water resources of relationships in glacial outwash aquifers. Water Resour. Res., 17(5): Nsukka and environs, Anambra State, Nigeria. Water Resources J. 1401 – 1408. NAH, 1[1&2]: 111-116. World Gazette (2011). Colourful map of Nigeria with 36 states. Fitts CR (2002). Groundwater Science. Elsevier Sci. Publi., the www.world–gazette.com. Netherlands. Freeze RA, Cherry JA (1979). Groundwater. Englewood Cliffs, New Jersey, Prentice-Hall, Inc. Frohlick RK, Kelly WE (1985). Estimates of specific yield with the geoelectric resistivity method in glacial aquifers. J. Hydrol., 97: 33- 44 . Huntly D (1986). Relations between permeability and electrical resistivity in granular aquifers. Ground Water. 24: 466-474. Keller GV, Frischknecht FC (1966). Electrical methods in Geophysical Prospecting. Pergamon Press. Kelly WE (1979). Geoelectric sounding for estimating aquifer hydraulic conductivity. Ground Water. 50(6): 420 – 425. Koefoed O (1979). Geosounding principles, I. Elsevier, Amsterdam. 276p.